The present invention relates to the general field of communication systems, and, in particular, to all-optical signal processors.
Currently, optical communication links are the preferred means of data transmission. Optical communication links have tremendous multi-terahertz bandwidth which allows transmission over, for example, several meters to transoceanic distances. The rapid accumulation of various propagation impairments (e.g., fading, scattering, diffraction of free-space optical connections, amplified spontaneous emissions, chromatic and polarization-mode dispersions and crosstalk for fiber-optic links), however, requires frequent termination of optical communication links by optical detectors, so that the signals are electronically regenerated and retransmitted by modulated laser sources. This so-called opto-electro-optical (OEO) regeneration is complex, bulky and extremely expensive. OEO regeneration for high-capacity optical links containing multiple wavelength-division-multiplexed (WDM) channels requires separate regenerators for each channel, and is thus especially complex and expensive.
Recent progress in high-speed detectors, optical amplifiers and modulation formats has enabled the un-regenerated reach distances of up to several thousand kilometers. Such long-haul transmission has been largely confined to point-to-point links which often times require OEO regeneration at the terminals interconnecting the links into the network. OEO regeneration is required after long-distance propagation because the signals lack the performance margin needed to accommodate considerable penalties such as crosstalk, polarization-dependent loss and spectral clipping by filters from multiple network elements used for switching and routing. In order to eliminate costly OEO regeneration, those skilled in the art have incorporated limited optical networking capabilities into the links (e.g., by means of optical add-drop multiplexers and cross-connects). However, without regeneration such measures cause dramatic reductions in the flexibility and scalability of the network and greatly increase the complexity of system management.
On the other hand, all-optical regeneration has been recognized as a potential enabler of future ultra-long reach high-bit-rate systems and all-optical packet-switched networks. All-optical regenerators with re-amplification and re-shaping (2R) capabilities have attracted particular attention because of their simplicity and robustness. In order to qualify as a viable alternative to current systems, all-optical regenerators must be easily scalable with the number of WDM channels. However, simultaneous multi-channel regeneration remains a formidable challenge because the operation of an all-optical regenerator fundamentally relies on strong nonlinear-optical effects which lead to debilitating interaction among the WDM channels, in particular by way of four wave mixing (FWM) and cross-phase modulation (XPM).
The operating principle of a single-channel 2R regenerator of the prior art is illustrated in FIG. 1, where: (1) dashed line is an input spectrum; (2) bold solid line is the self-phase modulation (SPM) broadened spectrum; and (3) dotted line with gray shading is the output spectrum selected by off-center filter. When an optical pulse (full bandwidth at half maximum Δν0) propagates through a nonlinear Kerr medium, such as highly nonlinear fiber (HNLF), its Fourier spectrum is broadened to width ΔνNL by the effect of SPM. For sufficiently large spectral broadening (i.e. when ΔνNL>>Δν0, the output bandwidth ΔνNL becomes approximately proportional to the input pulse power P0: ΔνNL∝P0. Therefore, the output spectral density, P0/ΔνNL becomes almost independent of the input power. By selecting a portion of the output spectrum with a narrow optical bandpass filter (OBPF), one produces the output pulses with relatively constant power independent of fluctuations of P0. This achieves regeneration of ONEs (note that the shape and duration of the output pulses are completely determined by the shape and bandwidth of the OBPF). In addition, the power of any (unwanted) electric field that accumulates in ZERO bit slots due to various transmission impairments and noise, is much smaller than the power of ONEs. Thus, this field propagates in a virtually linear regime and does not experience spectral broadening. Therefore, it is easily rejected by the OBPF that is sufficiently shifted from the input spectrum's center.
While the presence of non,zero dispersion in the fiber is not critical for the regeneration to occur, the regenerator's performance is improved in the presence of small negative dispersion which helps to flatten the ripples in the SPM-broadened spectrum.
In the case of multiple channels propagating in the nonlinear medium, however, the benefits of the 2R regeneration are overshadowed by enormous degradations coming from nonlinear interactions among the WDM channels, such as FWM and XPM. As a result, the prior art has failed to achieve the simultaneous 2R regeneration of multiple WDM channels.
While the cost, size and power consumption advantages of an all-optical regenerator are widely recognized for both fiber-based and free-spaced systems, prior art designs fail to meet that need. Prior art regenerators are single-optical-channel devices implemented on a channel-by-channel basis and are thus precluded from use in real networks.
What is needed therefore is an effective, cost-efficient method and system for deploying fully scalable and fully flexible all-optical networks that enable simultaneous processing of multiple WDM channels without converting them to the electrical domain.